The present invention relates to an apparatus and a method for controlling an internal combustion engine that has a fuel vapor treating apparatus, which collects fuel vapor in a fuel tank to a canister without releasing the fuel vapor into the atmosphere and purges the collected fuel vapor to the intake passage of the engine as necessary.
A typical internal combustion engine driven with volatile liquid fuel includes a fuel vapor treating apparatus. The fuel vapor treating apparatus has a canister for temporarily storing fuel vapor generated in a fuel tank. When necessary, fuel vapor collected by an adsorbent in the canister is purged to the intake passage of the engine from the canister through a purge passage, and is mixed with air drawn into the engine. The fuel vapor is combusted in the combustion chamber of the engine together with fuel injected from the injector. A purge control valve located in the purge passage adjusts the flow rate of gas (purge gas) containing fuel vapor to the intake passage.
In the above internal combustion engine, the air-fuel ratio of combustible gas mixture supplied to the combustion chamber is detected. The amount of fuel injected from the injector is controlled such that the detected actual air-fuel ratio matches with a target value.
To optimally control the air-fuel, the amount of fuel injected from the injector needs to be controlled by taking the amount of fuel vapor purged to the intake passage through the purge passage.
Typically, the amount of injected fuel is controlled in the following manner when the influence of fuel vapor is taken into consideration. First, a basic fuel injection amount (time) is computed based on parameters indicating the running state of the engine, such as the engine speed and the intake air amount. Then, a final fuel injection amount (time) is determined by adjusting the basic fuel injection amount with a air-fuel ratio feedback correction factor, an air-fuel ratio learning value, a purging air-fuel ratio correction factor, and a correction factors obtained based on the running states. The air-fuel ratio feedback correction factor corresponds the difference between the air-fuel ratio of the previous fuel injection relative to the stoichiometric air-fuel ratio. The air-fuel ratio feedback correction factor is used for permitting the air-fuel ratio in the current fuel injection to approximate the stoichiometric air-fuel ratio. The air-fuel ratio learning value is a correction factor that is learned and stored for each running state region based on the results of air-fuel ratio feedback control in different running state regions. Using the air-fuel ratio learning value improves the accuracy of the air-fuel ratio feedback control. The purge air-fuel ratio correction factor is obtained by considering the influence of the fuel vapor introduced into the intake passage to the air-fuel ratio. The purge air-fuel ratio correction factor is computed based on a purge rate and a vapor concentration learning value. The purge rate refers to a coefficient that represents the rate of the flow rate of purge gas introduced into the intake passage to the flow rate of intake air in the intake passage. The vapor concentration learning value refers to a coefficient that reflects the concentration of the vapor component in the purge gas. The product of the purge rate and the vapor concentration learning value is used as the purge air-fuel ratio correction factor for correcting the air-fuel ratio.
When the purge flow rate is abruptly changed, a response delay occurs due to the distance between the purge control valve and the combustion chamber. Accordingly, the purge flow rate is increased to a theoretical purge flow rate value, which corresponds to the actual opening degree of the purge control valve, after a delay. Thus, if the purge flow rate is abruptly changed, the actual purge rate is different from the theoretical purge rate, which corresponds to the theoretical value of the purge flow rate. Therefore, if the fuel injection amount is computed based on the theoretical purge rate, which corresponds to the theoretical purge flow rate value, the fuel injection amount would be insufficient or excessive, which causes the air-fuel ratio to be different from the stoichiometric air-fuel ratio.
To solve the above problems, Japanese Laid-Open Patent Publication No. 11-264351 discloses a controller that computes the flow rate of purge gas supplied to a combustion chamber by taking a response delay of purge flow rate due to the distance between a purge control valve and the combustion chamber. When the purge flow rate is abruptly changed, a change of the vapor concentration is estimated based on the rate of change of the purge flow rate.
When the purge flow rate is changed, the amount of fuel vapor separated from the adsorbent in a canister is changed accordingly. However, when the purge flow rate is abruptly increased, the amount of fuel vapor separated from the adsorbent is not quickly increased, which causes the separated fuel vapor to increase some time after the purge flow rate is increased. Therefore, when the purge flow rate is abruptly increased, the concentration of the fuel vapor in the purge gas is temporarily lowered. In the above mentioned publication, delay of separation of fuel vapor in the canister due to an abrupt increase of the purge flow rate is not taken into consideration. Therefore, when the purge flow rate is changed, the fuel vapor concentration cannot be accurately computed, which, in turn, causes an inaccurate computation of the fuel injection amount. The accuracy of the air-fuel ratio control is deteriorated, accordingly.
Accordingly, it is an objective of the present invention to provide an apparatus and a method for controlling an internal combustion engine that improves the accuracy of air-fuel ratio control when the purge flow rate is changed.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, an apparatus for controlling the air-fuel ratio of air-fuel mixture drawn into a combustion chamber of an engine is provided. An intake passage of the engine is connected to a canister by a purge line. The canister adsorbs fuel vapor generated in the fuel tank and permits the adsorbed fuel vapor to be separated. Gas containing fuel vapor is purged as purge gas from the canister to the intake passage through the purge line. The apparatus includes a purge controlling device, a sensor for detecting the air-fuel ratio of the air-fuel mixture, and a computer. The purge controlling device adjusts purge flow rate, which is the flow rate of the purge gas flowing through the purge line. The computer sets the amount of fuel supplied to the combustion chamber such that the detected air-fuel seeks a target air-fuel ratio. The computer computes the purge flow rate based on the state of the purge controlling device, and computes vapor concentration, which is the concentration of the fuel vapor contained in the purge gas, based on the difference between the detected air-fuel ratio and the target air-fuel ratio. In accordance with changes of the computed purge flow rate, the computer obtains a concentration correction value for correcting the computed vapor concentration. By taking into consideration of the difference between the time at which the purge flow rate is computed and the time at which purge gas having the computed flow rate is drawn into the combustion chamber, the computer corrects the computed vapor concentration by using the concentration correction value. The computer sets the fuel supply amount in accordance with the computed purge flow rate and the corrected vapor concentration.
The present invention may also be applied to a method for controlling the air-fuel ratio of air-fuel mixture drawn into a combustion chamber of an engine. An intake passage of the engine is connected to a canister by a purge line. The canister adsorbs fuel vapor generated in the fuel tank and permits the adsorbed fuel vapor to be separated. Gas containing fuel vapor is purged as purge gas from the canister to the intake passage through the purge line. The method includes the following steps: adjusting purge flow rate, which is the flow rate of the purge gas flowing through the purge line, with a purge controlling device; detecting the air-fuel ratio of the air-fuel mixture; computing the purge flow rate based on the state of the purge controlling device; computing vapor concentration, which is the concentration of the fuel vapor contained in the purge gas, based on the difference between the detected air-fuel ratio and the target air-fuel ratio; obtaining a concentration correction value in accordance with changes of the computed purge flow rate; correcting the computed vapor concentration by using the concentration correction value and by taking into consideration of the difference between the time at which the purge flow rate is computed and the time at which purge gas having the computed flow rate is drawn into the combustion chamber; and setting the amount of fuel supplied to the combustion chamber in accordance with the computed purge flow rate and the corrected vapor concentration such that the detected air-fuel ratio seeks a target air-fuel ratio.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.